A. Crumb Rubber Modified Asphalts
Crumb rubber obtained from recycled vehicle tires has been mixed with asphalt for various applications for at least 30 years to form various compositions generally referred to as “rubber modified asphalt” or “rubber asphalt cement.” In most of the previously known rubber modified asphalts or rubber asphalt cements, the crumb rubber remains in the form of solid particles after it has been combined with asphalt. In general, two (2) types of processes have been used to manufacture rubber modified asphalt or rubber asphalt cement for paving applications, namely the Wet Process and the Dry process.
In the Wet Process particles of crumb rubber are mixed with asphalt cement to form a crumb rubber/asphalt mixture wherein aromatic oils from the asphalt are absorbed into polymer chains of the rubber, thereby causing swelling of the rubber particles. This crumb rubber/asphalt mixture is then combined with aggregate (e.g., pulverized rock, stones or sand) and sometimes other components (e.g., slag, glass, recycled asphalt, etc.) to form a paving composition. The Wet Process typically requires special equipment for blending the crumb rubber and the asphalt to form the crumb rubber/asphalt mixture.
In the Dry Process, the crumb rubber is combined with aggregate (and sometimes other dry components of the paving composition to form a crumb rubber/aggregate dry mixture. Such crumb rubber/aggregate dry mixture is subsequently combined with asphalt to form the desired paving composition.
There have also been attempts to digest, devulcanize, liquefy or chemically modify the crumb rubber prior to or during blending with asphalt to provide a more homogenous, substantially liquid, rubber modified asphalt. These processes include a technique known as the “Terminal Blend Process” wherein crumb rubber is digested into the asphalt at a refinery. The Terminal Blend Process has been used in Texas since about 1995 and is generally understood to be incapable of incorporating as much crumb rubber into the asphalt as the Wet Process or the Dry process.
Various United States Patents have described methods for combining digested, devulcanized, liquefied or chemically modified crumb rubber prior to or during its mixture with asphalt, to form variations of rubber modified asphalt or rubber asphalt cement. For example, U.S. Pat. No. 5,270,361 (Duong et. al.) describes a process for making an asphalt composition which includes synthetic or natural rubber particles of up to one-half inch in thickness. Elemental selenium or an organo-selenium compound is added into the mixture to act as a substitute for the sulfur which is removed during the vulcanization process. The selenium or organo-selenium compound acts as a cross-linking agent. The composition is treated with pressurized air in a dehydrogenation reaction. In the dehydrogenation vessel, the dispersing device includes a pair of discs turning at 3600 rpm which promotes homogenization and acceleration of the dehydrogenation reaction. Elemental selenium or an organo-selenium compound is added into the homogenized composition and mixed in a static mixer. The asphalt composition is then recovered and stored in a container at about 150.degree. to 175.degree. C.
U.S. Pat. No. 4,609,696 (Wilkes) describes a rubberized asphalt composition wherein asphalt is combined with a hydrocarbon oil to provide a homogenized asphalt-oil mixture. Such asphalt-oil mixture is then combined with solvent-dispersed particulate rubber to provide a homogenous gel. This gel may then be emulsified by passing the asphalt-rubber-oil gel, with water, through a colloid mill.
Also, U.S. Pat. No. 4,430,464 (Oliver) describes a pavement binder composition in which rubber particles are digested in a bituminous material.
U.S. Pat. No. 4,588,634 (Pagen et al.) describes a roofing material using bitumen and ground tire rubber together with mineral stabilizer and an elastomeric polymer composition.
U.S. Pat. No. 5,334,641 (Rouse) describes a rubber modified asphalt for use as a paving compound which is formed by reacting ground rubber, of 50 mesh or finer, with paving grade asphalt and mixing the combination at 300.degree.-400.degree. F. The mixture reacts fully within 25 minutes or less to form a freely pouring mixture and can be held at normal asphalt working temperatures for at least 96 hours without degradation.
U.S. Pat. No. 4,485,201 (Davis) describes a method of modifying asphalt with a mixture of ground rubber and synthetic rubber. The compositions include oils and antioxidants in addition to asphalt and rubber. The processing takes place below 375.degree. F.
U.S. Pat. No. 4,085,078 (McDonald) describes a paving material formed by heating a mixture of paving grade asphalt and a non-oil resistant rubber to a temperature of about 360° F.-500° F. The asphalt rubber mixture is diluted with a diluent prior to reaction to form a gelled reaction product. McDonald, U.S. Pat. No. 4,069,182, also describes a hot gelled composition which can be applied to cracked or distressed pavements to repair such pavements. McDonald, U.S. Pat. No. 3,891,585, describes a similar hot gelled composition for repairing pavements.
U.S. Pat. No. 4,018,730 (McDonald) describes thixotropic emulsions prepared from rubber modified asphalt gel. The rubber modified asphalt gel is prepared by processing 20-50% particulate rubber with asphalt at temperatures above 350° F. The emulsion is then created by admixing an asphalt-soap to the rubber modified asphalt gel, thereby forming a thixotropic emulsion.
U.S. Pat. No. 3,919,148 (Winters et al.) describes an elastomeric paving material prepared from asphalt, rubber and an asphalt solvent to form a hot, thick, viscous, gelled composition.
U.S. Pat. No. 5,492,561 (Flanigan) describes a process for liquefying tire rubber known as the TRMACS process. In the TRMACS process, crumb rubber and the asphalt are heated to temperatures of approximately 500° F. under carefully controlled conditions. As a result, the crumb rubber becomes liquified, hydrogen sulfides and mercaptans evolve and the large insoluble polymer molecules of the rubber break into smaller thermoplastic molecules. These thermoplastic molecules are then adsorbed onto the maltenes or other components of the asphalt, providing a homogenous modified asphalt composition in which the rubber has been fully incorporated.
The entire disclosures of each of the above-mentioned United States Patents are expressly incorporated herein by reference.
B. Asphalt Emulsions
For many applications, asphalt is combined with water and a small amount of a surface active agent known as an emulsifier. Such mixture is then run through a device known as a colloid mill which shears the asphalt into tiny droplets. Due to the presence of the emulsifier, the tiny asphalt droplets remain dispersed throughout the water in a stable suspension known as an aqueous asphalt emulsion.
Asphalt emulsions are classified into three categories; anionic, cationic, or nonionic. The anionic and cationic classes refer to the electrical charges surrounding the asphalt particles. Asphalt emulsions have been used in a variety or products including driveway sealers, roofing repair materials, caulks, mastics, crack fillers and in paving materials such as seal coats, chip seals and slurry seals. In some instances, a polymer latex such as styrene-butadiene-rubber latex (SBR latex) is added to the aqueous asphalt emulsion to provide a “polymer modified” asphalt emulsion. Such polymer modified asphalt emulsions are desirable in certain applications such as slurry seal paving applications where it is desired for the asphalt emulsion to have improved physical properties such as better adhesion to particles of aggregate and/or to an underlying surface, better water resistance, better thermal stability, higher softening temperature, etc.
Asphalt Emulsion Slurry Seals are typically used for sealing and repairing roads, drive ways, parking lots and other black topped surfaces. Generally speaking, slurry seals are produced by combining an aqueous asphalt emulsion mixture (e.g., asphalt, water, emulsifier(s) and sometimes other additives (e.g., polymer modifiers, thickeners, surfactants, fillers) with aggregate (e.g., stones or sand) to form a slurry. This slurry is then dispensed onto a paved surface to form a new top layer or coating on the paved surface. In many applications, the aqueous asphalt emulsion mixture is pumped into a mixing apparatus known as a “pug mill” that is mounted on a slurry mixing truck. The aggregate is dispensed into the pug mill where it becomes combined with the liquid asphalt emulsion mixture to produce a slurry. The slurry then flows from the pug mill into a mixing box which travels just behind the slurry-mixing truck. The slurry then is spread from the mixing box onto the underlying paved surface.
Fillers, such as polymer fiber, paper or rubber particles (e.g., crumb rubber obtained from recycled vehicle tires) may also be contained in the slurry seal. Slurry seals that contain crumb rubber are often referred to as “rubberized” slurry seals. They differ from the above-described rubber modified asphalt or rubber asphalt cement in that the rubber particles are not incorporated into the asphalt prior to its emulsification, but rather are combined with the aqueous asphalt emulsion and other components to form an asphalt emulsion mixture which contains solid rubber particles. For example, U.S. Pat. No. 5,539,029 (Burris) and U.S. Pat. No. 5,811,477 (Burris, et. al.) describe asphalt emulsion mixtures that contain solid rubber particles as well as certain slurry seal compositions that are prepared by mixing such solid rubber particle-containing asphalt emulsion mixture with aggregate. In applications such as the above-described “truck mixed” application where it is necessary for the asphalt emulsion mixture to be passed through a pump, the inclusion of solid rubber particles (or other solid fillers such as polymer fibers, clay, etc.) can be problematic in that the rubber particles or filler particles can form clumps and can cause clogging of the pump, especially when the asphalt emulsion mixture is allowed to cool to ambient temperatures below about 55° F. Also, the inclusion of solid rubber particles (or other solid fillers such as polymer fibers, clay, etc.) in the aqueous asphalt emulsion mixture can cause increased wear to pumps, thereby requiring frequent replacement or rebuilding of the pumps.
Also, the inclusion of solid rubber particles in slurry seals can be problematic after the slurry seal has been applied. For example, in some applications the rubber particles may separate or “ravel” from the slurry seal coat.
There exists a need in the art for the development of methods for producing substantially liquid, non-thixotropic aqueous asphalt emulsions which incorporate recycled tire rubber (or break-down products of such tire rubber) but contain little or no (e.g., less than 2% by weight) solid particulate rubber. There is also a need in the art for the development of products such as driveway sealers, roofing repair materials, caulks, mastics, crack fillers and in paving materials such as seal coats, chip seals and slurry seals which contain substantially liquid, non-thixotropic aqueous asphalt emulsions which incorporate recycled tire rubber (or break-down products of such tire rubber) but contain little or no solid particulate rubber.